LEDs (light emitting diodes) are semiconductors that work like a diode yet their main feature is that they emit light.
Their conversion of electric power to light is so efficient that LED typically do not produce heat (very high powered LED still do): they are said to produce cold light.
Rugged Yet Simple to Destroy
LED are rugged and last longer and are more resilient to mechanical stress than most other light sources.
Low Internal Resistance
One pecularity makes them very delicate and sensitive, though: they have a low internal resistance.
When you connect an LED directly to a power source, it will almost instantly burn up. The LED low resistance behaves similar to a wire or a fuse (which have a low resistance, too):
When you short-circuit your power supply with them, for a fraction of a second a huge current flows and heats up and ultimately destroys both. The very same happens with LED.
Always Control Current
To operate LED safely, you must always limit the current that can flow through the LED. There are many ways to limit current. In hobbyist projects, a simple series resistor is the most popular one.
Identifying Anode and Cathode
Since LED are semiconductors and conduct current in one direction only, it is crucial to connect it to the correct polarity. The two legs of a LED are called anode (+) and cathode (-).
Forward Voltage, Reverse Voltage, and Breakdown Voltage
Any material has a breakdown voltage: at this voltage, it becomes conductive.
Semiconductors (like LED) have two breakdown voltages: the forward voltage is the breakdown voltage for the “normal” current that flows from + to - (in the correct or intended direction). Reverse voltage is the breakdown voltage when current flows the opposite direction.
When you connect a LED correctly to + and -, once the applied voltage exceeds the LED forward voltage, the LED starts to emit light. Typically, LED forward voltages are in the range of 1.6-4.0V, depending on their color and the materials they were made of.
When you connect a LED incorrectly and accidentally reverse polarity, the LED acts like a diode and will not conduct. Since LED were never designed to be diodes, no effort was made to optimize their reverse voltage. Compared to real diodes, the reverse voltage for LED is very low and around 5V.
So once the voltage of your incorrectly connected power exceeds 5V, the LED starts to conduct but cannot utilize the power to produce light. The power instead is converted to heat and ultimatively destroys the LED.
When you use a current-limiting series resistor or some other means of current limit, you are protected against accidental reverse voltage as well: even though the voltage may exceed the reverse voltage, only a very small current flows which is not powerful enough to cause real damage.
Standard Indicator LEDs
Indicator LED are cheap and simple to use. Many hobbyist projects use these.
Straw-Hat LED
Straw Hat LED have a shape resembling a straw hat, partially because they feature a built-in lens for a wide viewing angle.
Piranha / SuperFlux LED
Piranha LED are small and compact square LED with a relatively large light output with four pins (instead of two). They are also known as SuperFlux.
Dual Color LED
Dual color LED are really just two-in-one LED that internally consist of two regular LED in different colors. They are perfect for indicating two different states, i.e. funtional (green) and error (red).
RGB Color LED
RGB LED can produce any color: three internal LED in red, green and blue mix any other color.
Programmable LED
Once LED consist of more than one color, they become difficult to operate: each internal color LED has its own specific forward voltage and needs its own specific voltage.
Worse, LED strip multiply this effort: each of the three color LED in each of the connected RGB LED need to be carefully wired.
Programmable RGB LED come to the rescue by adding a tiny chip to each programmable RGB LED. Both color mixing and current control is managed by this chip. It is controlled by just one data pin that can be daisy chained to string up any number of programmable RGB LED.
Most commonly, the LED controllers are embedded in SMD LED like the type 5050 in the picture above.
Programmable LED are comprised of a LED and a controller chip. From the outside, you can only see the LED. This is why regular SMD LED are indistinguishable on first sight from programmable SMD LED.
The LED controller chip can essentially be embedded in any LED and are not restricted to LED strips. They are i.e. also available as single discrete LED*.
SMD LED
SMD (surface mounted device) LED are a special form factor and suitable for direct mounting (soldering) to PCB. This SMD form factor is available for any of the discussed LED types:
You get SMD versions of single color, RGB, and programmable LED.
3W High Performance LED (Generic)
LED can be used for illumination, too. These high performance LED turn much higher currents into light than the typical 10-20mA used by simple indicator LED.
COB LED
With dual color and RGB LED, you have seen multiple individual LED blended together. COB (Chip On Board) takes this a step further and mounts a large number of LED directly onto a substrate or circuit board.
The result are large surfaces of any shape or area, a cheap production method and very much homogenous light output.
The many different LED on a COB are partially connected in series and partially connected in parallel to design a particular desired total forward voltage. They often can be connected directly to voltages like 12-13V without the need of series resistors or constant voltage/constant current power supplies.
7-Segment Displays
Multiple LED are combined in 7-Segment displays. They are commonly used to display numeric information.
Dedicated controller IC make it simple to drive these displays via I2C or similar interfaces.
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(content created Mar 17, 2024 - last updated Mar 25, 2024)